Relatively inexpensive novelty items containing hidden images are produced in large quantities and are used in a variety of applications including game pieces, food packaging prizes, educational children's books and the like. Many of these items contain hidden images. The images may be developed or revealed by applying a liquid or solid developer or solvent to the hidden image area of the object, by removing an opaque coating over the image, by heating the image, by rubbing the image to rupture microcapsules containing color developer or colorant and the like. The disadvantage of many of these items is that the image is often blurry, weakly developed and/or does not contain vibrant well defined colors. Another disadvantage of such objects is that the developer or solvent is often toxic or hazardous and thus could be dangerous when used by unsupervised children or infants.
Color forming dyes which are applied to a substrate in the form of microcapsules have limited application for producing images as the microcapsules are relatively fragile and care must be taken to assure that the microcapsules are applied to the substrate without substantial breakage of the microcapsules. Scuffing the printed surface or exposing the printed surface to heat may prematurely rupture the microcapsules. Thus it is difficult to print latent or hidden images using microcapsules containing the reactive components by four color process techniques. Another disadvantage of microcapsules containing the color former or developer compound is that the developed colors are somewhat diluted by the presence of the microcapsule material and are less vibrant than colors developed in the absence of microcapsules.
In addition to the use of microcapsules, chromogenic compounds have been dispersed in a wax medium and applied to a substrate. The wax medium, however, may dilute or blur the color forming compounds and delay or retard the speed the image develops. As with the microcapsules described above, scuffing or exposing the wax medium to heat may cause premature release of the color forming compounds or developer. Accordingly, there remains a need for cost effect devices and methods for applying color forming compounds to substrates to produce full color images which may be developed using relatively safe image activation techniques.
Considering the foregoing, the disclosure provides a device for developing latent or hidden images includes a first substrate containing a hidden or latent image printed on a first surface thereof, the image containing one or more chromogenic compounds and a second substrate having a first surface containing a developer coating for developing the hidden or latent image. The developer coating contains a developer compound, a micro-encapsulated solvent, and an adhesive.
In another aspect the disclosure provides a method for making a device for developing a latent image or hidden image into a visible image. The method includes printing a latent or hidden image comprising a chromogenic compound on a first surface of a first substrate. A pressure-sensitive adhesive material is applied to a first surface of a second substrate. The material contains a developer compound, a micro-encapsulated solvent which is compatible with the chromogenic compound and developer compound, and an adhesive. Subsequent, interfacial pressure contact between the first surface of the first substrate and the first surface of the second substrate provide development of the latent or hidden image.
An important advantage of embodiments of the disclosure is that devices containing the hidden or latent images may be developed readily without the use of liquid developer compounds which may leak, stain, dry or which may contain toxic or harmful components. Another advantage is that black and white or full color images may be developed which have excellent resolution and/or possess vibrant colors. Still another advantage of embodiments of the disclosure is that the developed images are substantially permanent and cannot be readily altered or modified thereby improving the security of the developed images. Since a protective, substantially transparent film is used to apply the developer to the hidden image, there are no liquid developer pens or wax markers that come in contact with the image to smear the image. Hence, the hidden or latent image may be printed on a wider variety of substrates, including less porous substrates, films, and the like while conventional developer pens and wax markers are typically limited to use on porous substrates.